Not Applicable
The present invention is directed generally to optical transmission systems. More particularly, the invention relates to optical transmission systems including error correction and protection capability for use in optical communication systems.
Optical transmission systems employ optical transmitters to transmit information as optical signals via guided or unguided transmission media, typically optical fiber, to optical receivers. The optical fiber attenuates the optical signals and accumulates optical noise resulting from signal attenuation and other degradation mechanisms, such as non-linear effects, which degrades the quality of the optical signals during transmission.
The distance that the transmitters and the receivers can be separated in a transmission system depends upon the amount of signal degradation that occurs during transmission. The amount of signal degradation and the resulting number of errors that occur in the optical signal depends upon the relative intensities of the optical signal and the noise in the system, known as the optical signal to noise ratio (xe2x80x9cOSNRxe2x80x9d).
Optical amplifiers can be distributed along the transmission fiber to extend the distance between transmitters and receivers by amplifying the signal intensity to overcome signal attenuation in the fiber. However, optical amplifiers also amplify the optical noise in the fiber, which continues to accumulate in the transmission fiber and is amplified at successive amplifiers, further degrading the OSNR during transmission.
The accumulation of noise in optical systems often requires that the optical signals be regenerated along the transmission path between the origin and the destination before signal degradation introduces an unacceptable number of errors into the optical signals. Optical signal regeneration generally is performed by receiving and converting the optical signals into electrical signals. The electrical signals are processed, such as by retiming, reshaping, amplifying etc., to eliminate signal distortion introduced by the various degradation mechanisms in the transmission fiber. The regenerated electrical signals can then retransmitted as optical signals.
The transmitters, receivers, and associated equipment required to regenerate signals are often one of the largest component expenses in the optical system and along with the required real estate and facilities comprise a substantial portion of the optical system startup and operating costs. As such, it is desirable to extend the distance between transmitters and receivers to decrease the overall system cost, without further degrading the performance of the system.
Forward error correction (xe2x80x9cFECxe2x80x9d) has been used to extend the distance between the transmitters and receivers without incurring a commensurate increase in the error rate and decrease in the quality of service. FEC is implemented by encoding the information to be transmitted, such that the information can be decoded upon receipt and the decoding process will correct a limited number of errors introduced during transmission of the information.
Because systems employing FEC can correct a limited number of errors, higher levels of signal degradation, and lower OSNR, can be tolerated before an unacceptable number of errors pass through the system. FEC provides a dual benefit in that FEC increases the system performance by decreasing the error rate of the system and decreases the system cost by allowing for greater distances between transmitters and receivers in the system.
While FEC can extend the transmission distance of the system, FEC encoding adds encoding bits to signal and increases the overall bit rate of the signal. The increased bit rate associated with FEC can increase the degradation of the optical signals by various mechanisms, thereby requiring that the transmitters and receivers be more closely spaced.
As the demand for transmission capacity continues to grow, there is an increasing need to efficiently use the available transmission capacity and protect the information being transported through the systems. In addition, the increased amount of traffic being carried on each fiber places increased importance on the ability to effectively protect the information, because each failure results in higher revenue losses for service providers. Accordingly, there is a need for optical transmission systems and protection schemes that have increased flexibility and reliability for use in optical communication systems.
The present invention addresses the need for optical transmission systems, apparatuses, and methods having increased flexibility and reliability. Optical systems of the present invention include at least one optical transmitter configured to provide forward error correction (xe2x80x9cFECxe2x80x9d) encoding of information to be transmitted to corresponding optical receivers that provide for FEC decoding of the information. In various embodiments, the receiver threshold, amplitude and/or temporal, for distinguishing between ones and zeros in the received information is adjusted based on the number of errors, either one or zero, detected during decoding of the transmitted information. In particular, the receiver threshold is periodically or continually adjusted to maintain the number of one errors and the number of zero errors within a specific relationship. For example, a threshold adjustment circuit can be configured to adjust the receiver threshold such that the number of one errors equals the number of zero errors or is maintained within a given difference and total number criteria. In addition, the receiver filters, distortion compensation circuits, and signal launch powers can be adjusted to maintain a relationship between the number of one errors, zero errors, and total errors.
A network management system (xe2x80x9cNMSxe2x80x9d) can track the number of one, zero, and total errors being corrected, either locally at the network element level or at higher network management levels. The system can be configured to perform protection switching of the information to an alternate communication path when a certain threshold for the number of errors corrected by FEC is exceeded. The protection switching can be performed automatically or upon prompting by the NMS. For example, automatic switching can initiated when a rapid increase in errors, above an error burst threshold, is detected that requires immediate intervention. Conversely, the NMS can notify the network operator of trends of increasing errors being corrected by the system. These embodiments allows protection switching to be planned and performed before degradation along the communication path actually results in diminished system performance.
In various embodiments, the transmitters and receivers are controlled to implement various levels of FEC encoding and decoding, respectively. The level of FEC encoding, and hence the bit rate of the information including FEC overhead, can be used to balance the error rate with the transmission distance to optimize the performance of the system. For example, the amount of FEC encoding can be reduced when few or no errors are being detected, thereby lowering the bit rate of the signal and extending the transmission distance of the information signal. Conversely, the FEC encoding level can be increased, when larger numbers of errors are being detected and corrected. The variable FEC technique provides an adapting coding technology that increases the level of system optimization.
Also, the FEC encoding can be configured to transmit additional information along with the information being encoded. For example, low frequency tones can be included in the FEC encoding to transmit low bandwidth information, such as channel identification, without having to receive and decode the information. Higher bandwidth information, such as system supervisory information and low bit rate payload information, can also be included in the FEC encoding as recovered at the receivers or signal monitors in the system. For example, a reboot command for a central processor (xe2x80x9cCPxe2x80x9d) can be encoded along with the information being transmitted to the receiver. When the reboot command is decoded by the receiver and provided to the CP, the command will cause it to reboot.